Controllable Synthesis of Bi2Te3 Intermetallic Compounds with Hierarchical Nanostructures via Electrochemical Deposition Route Gao-Ren Li,*,†,‡ Fu-Lin Zheng,† and Ye-Xiang Tong*,† MOE of Key Laboratory of Bioinorganic and Synthetic Chemistry/School of Chemistry and Chemical Engineering/Institute of Optoelectronic and Functional Composite Materials, Sun Yat-Sen UniVersity, Guangzhou 510275, and State Key Lab of Rare Earth Materials Chemistry and Applications, Beijing 100871, China
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 4 1226–1232
ReceiVed August 20, 2007; ReVised Manuscript ReceiVed January 19, 2008
ABSTRACT: Bismuth-tellurium intermetallic compounds have proven to be one of the best thermoelectric materials with the highest figure of merit and have proven to be very useful for devices operating at room temperature. Here, we report that the Bi2Te3 intermetallic compounds with novel hierarchical nanostructures can be successfully synthesized by an electrochemical deposition route, which presents a simple, quick, and economical method for the preparation of Bi2Te3 intermetallic compounds and the controllable growth of hierarchical nanostructures. Cyclic voltammetry was used to study the electrochemical reactions relevant to the growth of Bi2Te3 intermetallic compounds, and the electrochemical formation process of Bi2Te3 intermetallic compounds was also investigated in this paper. The synthetic parameters in this research allowed various structural manipulations for Bi2Te3 intermetallic compounds, and the formation mechanisms of Bi2Te3 intermetallic compounds with hierarchical nanostructures were proposed. The concentration ratio of Bi(NO3)3 to Na2TeO3 was a key factor that affected the morphologies and structures of Bi2Te3 intermetallic compounds. These prepared novel hierarchical Bi2Te3 nanostructures are desired low dimensional thermoelectric building blocks for possibly achieving a high thermoelectric figure of merit.
1. Introduction Thermoelectric materials are special types of semiconductors that, when coupled, function as a “heat pump”. The converters from heat to electricity have no moving parts, and no pollutant is released to the environment. Therefore, thermoelectric materials are extremely reliable, scalable, and ideal for miniature power generation. They are strongly advantageous as compared to conventional energy technologies, although current thermoelectric devices have a low conversion efficiency of about 10%. However, if significantly improved thermoelectric materials could be developed, many applications of thermoelectric devices will be envisaged. For example, thermoelectric devices may possibly be used to convert huge amounts of industrial waste heat into electricity, and they could also make power generators in cars by utilizing heat from the exhaust gases. Novel materials with better thermoelectric performances are being investigated and will improve the efficiency of thermoelectric devices, decrease their operation costs, and widen the range of their applications. The efficiency of thermoelectric devices is a function of the nondimensional figure of merit, ZT, of the material, and Z can be expressed as follows (where T is the absolute temperature):1,2 Z ) σR2/k, where σ is the electrical conductivity, R is the thermoelectric power or Seebeck coefficient, and k is the thermal conductivity. Therefore, the challenge for realizing superior thermoelectric materials lies in the improvement of the thermoelectric figure of merit, that is, simultaneous increase of thermoelectric power, increase of electrical conductivity, and decrease of thermal conductivity.3 Bismuth-tellurium is commonly used in commercially available Peltier cooling devices and has been proven to be one of the best thermoelectric materials with the highest figure of merit and to be very useful for devices operating at room temperature.4–6 * To whom correspondence should be addressed. E-mail: (G.-R.L.) ligaoren@ mail.sysu.edu.cn. (Y.-X.T.)
[email protected]. † Sun Yat-Sen University. ‡ State Key Lab of Rare Earth Materials Chemistry and Applications.
However, the small improvements in thermoelectric properties of Bi2Te3 since 1954 have limited their transition to commercial applications.7,8 Recently, much attention has been focused on the factorial enhancements in ZT by introducing nanoscopic geometrical confinements in one, two, or three dimensions or by tailoring the nanoparticle shape because theoretical predictions and experimental investigations have suggested that nanostructuring thermoelectric materials can be considered as a successful strategy to gain factorial enhancements in ZT due to both a high density of states and an increased phonon scattering or reduced lattice thermal conductivity in nanosystems.9–15 Although the synthesis and assembly of bismuth telluride nanostructures, such as nanorods,16 nanowires,4 nanoparticles,1 and nanosheets,3 have been widely explored, the growth of novel nanostructures of bismuth telluride intermetallic compounds with well controllable crystalline morphology via a simple, quick, and economical method still needs to be well-exploited. Various methods have been developed for synthesizing thermoelectric materials, such as sputtering,17 evaporation,18,19 metal-organic chemical vapor deposition (MOCVD),20,21 electrochemical deposition,22–31 template-assisted growth,4,24,26,31 and high-temperature solution synthesis.3,32Among these techniques, electrochemical deposition has shown the powerful ability to control the crystallization engineering, and it presents a simple, quick, and economical method for the preparation of large area thin films and has the advantage of allowing the controllable growth of patterned nanostructures. In this paper, we first report the electrochemical control synthesis of the novel hierarchical nanostructures of bismuth telluride intermetallic compounds, which offers a potential promise for realizing high-ZT thermoelectric nanostructures.
2. Experimental Section In our experiments, a simple three-electrode cell was used. A saturated calomel electrode (SCE) was used as the reference electrode that was connected to the cell with a double salt bridge system; a graphite rod with about 4.0 cm2 was used as the auxiliary electrode,
10.1021/cg700790h CCC: $40.75 2008 American Chemical Society Published on Web 03/14/2008
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Figure 1. Cyclic voltammograms of Pt electrodes in solutions of (a) 0.005 M Bi(NO3)3 + 0.01 M tartaric acid + 1.00 M HNO3, (b) 0.01 M Na2TeO3 + 0.01 M tartaric acid + 1.00 M HNO3, (c) 0.01 M Bi(NO3)3 + 0.005 M Na2TeO3 + 0.01 M tartaric acid + 1.00 M HNO3, and (d) 0.005 M Bi(NO3)3 + 0.01 M Na2TeO3 + 0.01 M tartaric acid + 1.00 M HNO3 at room temperature (scan rate, 100 mV/s). and a pure copper foil was used as the working electrode. Before deposition, the Cu foil was cleaned ultrasonically in 0.1 mol/L HCl, distilled water, and acetone and then rinsed in distilled water again. The electrolyte was composed of Bi(NO3)3, Na2TeO3, tartaric acid, and HNO3. The potentiostatic electrodeposition was performed at -0.10, -0.15, and -0.20 V, respectively, as these potential values are within the common diffusion-controlled region between all used solutions.21 Typical electrodeposition duration was around 60 min at room temperature. Finally, the deposited films were removed from the electrolyte and rinsed with distilled water and acetone. The electrodeposited bismuth telluride intermetallic compound films were imaged with scanning electron microscopy (SEM, JSM-6330F). An Oxford Instrument’s INCA energy-dispersive spectrometer (EDS) was employed to analyze the chemical composition. X-ray diffraction (XRD) (D/Max-IIIA, Rigaku, Japan) (Cu KR radiation, λ ) 0.15405980 nm) was used to determine the structures of the deposits.
3. Results and Discussion The electrochemical reactions relevant to the growth of bismuth telluride intermetallic compounds were studied by cyclic voltammetry. Bi(NO3)3 and Na2TeO3 were dissolved in nitric acid to form the oxide cations BiO+ and HTeO2+, respectively. Figure 1a shows the cyclic voltammogram (CV) of the Pt electrode in a solution of 0.005 M Bi(NO3)3 + 0.01 M tartaric acid + 1.0 M HNO3. One cathodic wave (1) was observed, and its peak potential appeared at about -0.07 V. This cathodic wave corresponds to the electroreduction of BiO+, namely, 2H+ + BiO+ + 3e- f Bi + H2O. Figure 1b shows the CV of the Pt electrode in a solution of 0.01 M Na2TeO3 + 0.01 M tartaric acid + 1.0 M HNO3. One cathodic wave (3) was also observed, and its peak potential appeared at about -0.27 V. This cathodic wave corresponds to the electroreduction of HTeO2+, namely, 3H+ + HTeO2+ + 4e f Te + 2H2O. Figure 1c shows the CV
of the Pt electrode in a solution of 0.01 M Bi(NO3)3 + 0.005 M Na2TeO3 + 0.01 M tartaric acid + 1.0 M HNO3. These results of CV are similar to the findings of Lecuire et al. when BiO+ is at a higher concentration than HTeO2+.9 Upon reduction, only one cathodic wave (5) centered at about -0.12 V was observed, after which the electrode appears dark gray because of the deposition of bismuth telluride intermetallic compounds. So this cathodic wave corresponds to the codeposition of bismuth telluride intermetallic compounds. On the anodic scan, an anodic wave is initially observed at about 0.13 mV, which corresponds to the anodic stripping of Bi, and then a sharp anodic stripping wave is seen at 0.46 mV that is attributed to the anodic stripping of Bi2Te3. However, the anodic wave corresponding to the stripping of Bi is unobserved in Figrue 1d when the concentrations of BiO+ are lower than those of HTeO2+ in deposition solution. This may be attributed to all of the deposited Bi joining in the formation of Bi2Te3 intermetallic compounds. The electrochemical formation process of bismuth telluride intermetallic compounds on Cu substrate was investigated here, and two possible formation routes were illuminated as following. When the electrodeposition was carried out with a higher potential (>-0.1 V), BiO+ can not be electrochemically reduced on cathode, but the electrochemical reduction of HTeO2+ to Te2- on the surface of cathode can happen, which will result in the formation of bismuth telluride intermetallic compounds during electrodeposition. This formation process of bismuth telluride intermetallic compounds can be expressed as eqs 1 and 2.23 On the other hand, when the electrodeposition is carried out with lower potential ( 0)].
out using X-ray photoelectron spectroscopy (XPS, ESCALAB 250), and the XPS spectra of the bismuth 4f and tellurium 3d regions for bismuth telluride nanosheets are shown in Figure 5. The Bi 4f7/2 peak at about 158.7 eV and 4f5/2 peak at about 164.1 eV are observed in Figure 5a. The Te 3d3/2 and 3d5/2 peaks at about 586.2 and 575.7 eV are observed in Figure 5b. The results of XPS further proved that Bi2Te3 intermetallic compounds were prepared. The shape of the primary bismuth telluride particles was dominated by the intrinsic crystal properties (i.e., platelet seed).3 The rate of crystal growth of bismuth telluride along the topbottom crystalline planes in the anisotropic bonding environment
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Figure 8. SEM images of bismuth telluride intermetallic compounds prepared in a solution of 0.0025 M Bi(NO3)3 + 0.005 M Na2TeO3 + 1.0 M HNO3 + 0.1 M tartaric acid with a deposition potential of –0.15 V. (a) 23000× and (b) 45000×.
is much greater than that along c-axis that is perpendicular to these top-bottom planes.3 However, as we all know, the crystalline facets tend to develop on the low-index planes to minimize the surface energy during growing, so this factor also obviously affects the crystal growth. When tartaric acid that was used as a stabilizing agent was added into the deposition solution, it is well-known that the energetic difference between the top-bottom facets (large surfaces) and the side facets, which are parallel to the c-axis (small surfaces), was further enlarged as the capping of tartaric acid on the surfaces of bismuth telluride nanocrystals, which favors the formation of thicker nanosheets.3 However, in this experiment, the concentration ratio of Bi(NO3)3 to Na2TeO3 is found to be a key factor that affects the morphologies of bismuth telluride intermetallic compounds. When the concentration of Bi(NO3)3 is larger or equal to that of Na2TeO3 in deposition solution, the dendritic structures constructed with nanoparticles will be formed. Also, it was found that the nanoparticles are smaller with decreasing the concentration ratio of Bi(NO3)3 to Na2TeO3. For example, the nanoparticles in dendritic structures prepared in a solution of 0.005 M Bi(NO3)3 + 0.005 M Na2TeO3 + 0.01 M tartaric acid + 1.00 M HNO3 are smaller than those prepared in a solution of 0.01 M Bi(NO3)3 + 0.005 M Na2TeO3 + 0.01 M tartaric acid + 1.00 M HNO3 as shown in Figure 6. The nanosheets of bismuth telluride intermetallic compounds can be successfully obtained when the concentration of Bi(NO3)3 is smaller than that of Na2TeO3 in the deposition solution, as shown in Figure 2. Therefore, the surface morphologies of these bismuth telluride intermetallic compounds can be easily tuned by varying the concentration ratio of Bi(NO3)3 to Na2TeO3. The effect mechanism of the concentration ratio of Bi(NO3)3 to Na2TeO3 in deposition solution can be explained as follows. As we all know, when the concentration of Na2TeO3 is higher than that of Bi(NO3)3 in the deposition solution, the obtained deposits will be favored to form Bi2Te3 intermetallic compounds as the atom number of Te is more than that of Bi in Bi2Te3. As mentioned before, Bi2Te3 has a rhombohedral structure with hexagonal cells of a ) 0.4384 nm and c ) 3.045 nm, which is favored to form plateletlike structures.3,36 However, the Bi2+xTe3-x (x > 0) intermetallic compounds will be formed when the concentration of Na2TeO3 is lower than that of Bi(NO3)3, which will lead to the formation of bismuth telluride nanoparticles. The XRD pattern of Bi2+xTe3-x (x > 0) intermetallic compounds is shown in Figure 7, and the peak of Bi(113) is observed. We further carried out some experiments to prove the above conclusion was right. The hierarchical bismuth telluride nanostructures were successfully prepared as shown in Figure 8 when the electrodeposition was carried out in solution of 0.0025 M
Figure 9. (a–c) Schematic illustration for the formation of hierarchical bismuth telluride nanostructures.
Bi(NO3)3 + 0.005 M Na2TeO3 + 0.1 M tartaric acid + 1.0 M HNO3 with a potential of -0.15 V. From Figure 8, it can be clearly seen that the construction units of these interesting and complex structures consist of large numbers of nanosheets. The backbones of hierarchical bismuth telluride nanostructures are the nanosheets with a thickness of about 20 nm as shown in Figure 8b. The branches are also consist of nanosheets, and their thickness is about 12 nm. It should be noted that the subbranches are also formed on the original branches, and the thickness of these subbranches is about 8 nm. The thickness of nanosheets was gradually decreased from the backbone to the original branches and subbranches. Therefore, the formation process of hierarchical bismuth telluride nanostructures should comply with the following rules: First, the backbones are formed, then the original branches are formed on the backbones, and finally the subbranches are formed on the original branches. A schematic illustration for the possible formation process of hierarchical bismuth telluride nanostructures is shown in Figure 9. Figure 10a shows an SEM image that displays some similar hierarchical bismuth telluride nanostructures prepared in 0.005 M Bi(NO3)3 + 0.01 M Na2TeO3 + 0.1 M CH3COONH4 + 1.00 M HNO3 with a deposition potential of -0.15 V for 60 min, and they also consist of large numbers of nanosheets. These hierarchical nanostructures of bismuth telluride intermetallic compounds are connected with each other. A close examination of these hierarchical nanostructures also indicates that the original nanosheet branches grow out of a nanosheet backbones, and the nanosheet subbranches grow out of the original nanosheet branches. The thicknesses of the nanosheets of backbones are about 60 nm, and those of the nanosheets of original branches are about 10 nm. However, the hierarchical nanostructures consisting of large numbers of nanorods of bismuth telluride intermetallic compound were occasionally obtained when the electrodeposition was carried out in solution of 0.005 M Bi(NO3)3 + 0.01 M Na2TeO3 + 0.01 M tartaric acid + 1.00 M HNO3, and this is shown in Figure 10b. The diameter of the nanorod axis is about 60-80 nm, and those of the nanorods attaching on the principal axis are about 30-60 nm. The
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Figure 10. SEM images of (a) the hierarchical nanostructures of bismuth telluride intermetallic compounds connected with each other and (b) the hierarchical nanostructures of nanorods.
Figure 11. (a–c) SEM images of bismuth telluride intermetallic compounds prepared in a solution of 0.0025 M Bi(NO3)3 + 0.005 M Na2TeO3 + 1.0 M HNO3 + 0.1 M citric acid with a deposition potential of –0.10 V.
Figure 12. SEM images of bismuth telluride intermetallic compounds prepared in a solution of 0.0025 M Bi(NO3)3 + 0.005 M Na2TeO3 + 1.0 M HNO3 + 0.1 M citric acid with a deposition potential of –0.20 V. (a) 8000× and (b) 23000×.
formation of bismuth telluride nanorods may be attributed to the energetic difference between the top-bottom facets, and the side facets of bismuth telluride crystals are enlarged, which leads to the crystal growth of bismuth telluride intermetallic compounds on the top-bottom facets along the c-axis according to the above discussion. Finally, this will lead to the formation of the hierarchical nanostructures that consist of nanorods. When citric acid was added into the deposition solution, namely, the electrodeposition was carried out in a solution of 0.0025 M Bi(NO3)3 + 0.005 M Na2TeO3 + 0.10 M citric acid + 1.0 M HNO3, and the deposition potential was controlled at -0.10 V; some interesting bismuth telluride clusters were obtained as shown in Figure 11. From Figure 11b,c, it can be
observed that these clusters are accumulated by the cone-shaped cages that also consist of nanosheets. The thicknesses of the sides of these cone-shaped cages are about 30-50 nm as shown in Figure 11c. However, when the deposition potential was negatively shifted to -0.20 V, the clusters of cone-shaped cages evolved to nanoporous spheres as shown in Figure 12. The diameters of these pores are about 200-600 nm, and the thicknesses of the walls are about 40 nm. This evolvement of nanoporous spheres from the clusters of cone-shaped cages can be explained as follows. With the deposition potential negatively shifting, which will result in the faster electroreductions of BiO+ and HTeO2+ ions, the deposition rate of bismuth telluride intermetallic compounds is increased, and a larger quantity of
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nucleates will be produced. To keep their surface energy lower, these deposited nucleates will aggregate together and adopt an appropriate structure. It is well-known that the primary driving force for simple particle growth comes from the reduction in surface energy, and the further reduction in surface energy will drive the morphology evolution due to the minimization of high surface energy faces.37,38 Therefore, we finally obtained the spherelike foam nanostructures, which have the lowest surface energy, with the deposition potential negatively shifting.
4. Conclusions In summary, here, we first report that Bi2Te3 intermetallic compounds with novel hierarchical nanostructures can be successfully synthesized via an electrochemical deposition route. The electrochemical deposition presents a simple, quick, and economical method for the preparation of bismuth telluride intermetallic compounds and has the advantage of allowing the controllable growth of hierarchical nanostructures. Furthermore, our synthetic parameters allow further structural manipulation. The experimental results showed that the concentration ratio of Bi(NO3)3 to Na2TeO3 in deposition solution played a key role in the tunable synthesis of hierarchical nanostructures of bismuth telluride intermetallic compounds. The effects of deposition potentials and additives on Bi2Te3 hierarchical nanostructures were also very obvious. These prepared novel hierarchical bismuth telluride nanostructures may have important potential applications in enhancing the thermoelectric performance. Acknowledgment. This work was supported by the Natural Science Foundations of China (Grant Nos. 20603048 and 20573136), the Natural Science Foundations of Guangdong Province (Grant Nos. 06300070, 06023099, and 04205405), and the Foundations of Potentially Important Natural Science Research and Young Teacher Starting-up Research of Sun YatSen University.
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